Forbid include of soem inet6 header files from wrong place

Forbid include of soem inet6 header files from wrong place

  KAME put INET6 related stuff into sys/netinet6 dir, but IPv6
  standard API(RFC2553) require following files to be under sys/netinet.
    netinet/ip6.h
    netinet/icmp6.h
  Now those header files just include each following files.
    netinet6/ip6.h
    netinet6/icmp6.h

  Also KAME has netinet6/in6.h for easy INET6 common defs
  sharing between different BSDs, but RFC2553 requires only
  netinet/in.h should be included from userland.
  So netinet/in.h also includes netinet6/in6.h inside.

  To keep apps portability, apps should not directly include
  above files from netinet6 dir.
  Ideally, all contents of,
    netinet6/ip6.h
    netinet6/icmp6.h
    netinet6/in6.h
  should be moved into
    netinet/ip6.h
    netinet/icmp6.h
    netinet/in.h
  but to avoid big changes in this stage, add some hack, that
    -Put some special macro define into those files under neitnet
    -Let files under netinet6 cause error if it is included
     from some apps, and, if the specifal macro define is not
     defined.
     (which should have been defined if files under netinet is
     included)
    -And let them print an error message which tells the
     correct name of the include file to be included.

  Also fix apps which includes invalid header files.

Approved by: jkh

Obtained from: KAME project